The present invention relates to a visible light emitting diode with higher output and longer service life.
In recent years, there have been increasing demands on light emitting diode ("LED"), which is now used instead of incandescent lamp because of its improved performance characteristics.
The increase of the replacement of the incandescent lamp by the LED is attributable to the fact that:
(1) the service life of the incandescent lamp is by about one year;
(2) the light emitted from the incandescent lamp has a continuous spectrum, and a filter is required because beautiful single luminous color ranging from blue to red cannot be obtained from the incandescent lamp; and
(3) it is not possible to produce the incandescent lamp in a small size.
On the other hand, LED can solve these problems. However, there are now more strict demands on the quality of LED.
LED is manufactured from an epitaxial wafer, which comprises single crystal substrate of a compound of an element of III-V family of zinc blende (sphalerite) type such as gallium phosphide (GaP), gallium arsenide (GaAs), etc., on which a layer of a desired zinc blende type III-V family compound crystal is epitaxially grown.
This is because the crystals currently available for the substates have many defects and are low in purity, and it is difficult to use them as LED. For this reason, a layer having such a composition as to provide a desired light emitting wavelength is epitaxially grown on the substrate.
As the material for the epitaxial growth layer, a gallium phosphide arsenide ternary mixed crystal layer is used for the manufacture of visible light LED from red to yellow. Also, a gallium phosphide epitaxial layer is used for the manufacture of a green LED.
As a crystal plane of these single crystal substrates, a plane has been used, which is by several degrees tilted from a (100) plane or a (100) plane toward the direction of 111!. In the zinc blende type crystal, the (100) plane has a four-fold axis of symmetry. As a result, it is easier to cleave it in rectangular or square shape or to grow it empirically.
Also, by being tilted by several degrees from the (100) plane toward 111!, i.e. by providing the so-called "off-angle", it is possible to form the growth step intentionally on the crystal growing surface and to extremely improve surface conditions of the epitaxial growth layer.
In recent years, with the extension of the applications of LEDs for equipment and devices, LEDs are more frequently used in outdoor applications.
When LEDs are used outdoors, light emitting output must be as high as possible. Further, as for outdoor application, environmental conditions are so severe that light output may decrease within a shorter time than it does in indoor application. Also, in case of a display plate using a large number of LEDs, unevenness in color may occur due to variations in the light emitting output. To overcome these problems, LED must be of higher output and of longer service life.
In a conventional filament type electric bulb, service life is defined as the duration until the filament is exhausted and light is no more emitted. On the other hand, the service life of LED is the duration until light emitting output of LED is decreased to a certain percentage of the initial value.
Having made strenuous efforts to solve the above problems, the present inventors have found that light output and service life of a LED depend on crystal plane orientation (Miller indices) of the substrate.
The above object can be attained by a light emitting diode according to the present invention, which comprises an epitaxial wafer having a monocrystal substrate of a compound of an element of III-V family with crystal structure of zinc blende type where a gallium phosphide or a gallium phosphide arsenide mixed crystal epitaxial layer is grown, whereby the surface of said substrate has one of the following crystallographic crystal plane orientations:
(1) a plane tilted by 5 to 16.degree. from a (100) plane toward 010!, 001!, 0-10! or 00-1!;
(2) a plane tilted by 5 to 16.degree. from a (-100) plane toward 010!, 001!, 0-10! or 00-1!;
(3) a plane tilted by 5 to 16.degree. from a (010) plane toward 100!, -100!, 001! or 00-1!;
(4) a plane tilted by 5 to 16.degree. from a (0-10) plane toward 100!, -100!, 001! or 00-1!;
(5) a plane tilted by 5 to 16.degree. from a (001) plane toward 100!, -100!, 010! or 0-10!; or
(6) a plane tilted by 5 to 16.degree. from a (00-1) plane toward 100!, -100!, 010! or 0-10!.
All of these crystal plane orientations are crystallographically equivalent crystal plane orientations.
As the monocrystal substrate of a compound of III-V family element of zinc blende type, a GaAs substrate, or more preferably a GaP substrate is used. This is because GaP substrate has a smaller difference in composition from the epitaxial layer.
It is preferable that crystal plane orientation of these substrates is one of the following crystal plane orientations because high light emitting output and longer LED service life are ensured:
(1) a plane tilted by 11 to 15.degree. from a (100) plane toward 010!, 001!, 0-10! or 00-1!;
(2) a plane tilted by 11 to 15.degree. from a (-100) plane toward 010!, 001!, 0-10! or 00-1!;
(3) a plane tilted by 11 to 15.degree. from a (010) plane toward 100!, -100!, 001! or 00-1!;
(4) a plane tilted by 11 to 15.degree. from a (0-10) plane toward 100!, -100!, 001! or 00-1!;
(5) a plane tilted by 11 to 15.degree. from a (001) plane toward 100!, -100!, 010! or 0-10!; or
(6) a plane tilted by 11 to 15.degree. from a (00-1) plane toward 100!, -100!, 010! or 0-10!.
If the direction of tilt angle is other than the above, light output and service life of LED are not improved. If tilt angle is out of the above range, it is not preferable because light output and service life of LED are not improved.
For epitaxial growth of a GaP layer or gallium phosphide arsenide mixed crystal layer, i.e. GaAs.sub.1-x P.sub.x (0.45&lt;x&lt;1), using these substrates, liquid phase growth method may be used. More preferably, vapor phase growth method, e.g. halogen transport method, is used. This is because it is difficult to change the composition smoothly by the liquid phase growth method.
In the vapor phase growth method, a graphite or a quartz holder is placed in a reactor made of quartz, and material gas is introduced to heat for epitaxial growth.
In case a gallium phosphide arsenide mixed crystal (GaAs.sub.1-x P.sub.x ; 0.45&lt;x&lt;1) is epitaxially grown on a GaP substrate, lattice mismatch is found in lattice constant of the substrate and the epitaxial layer. To decrease crystal defects due to lattice mismatch of a light emitting layer having a certain composition, a layer called "graded layer" with a composition gradually changing from the substrate to the light emitting layer is provided between the substrate and the epitaxial layer.
Also, in order to improve light emitting output when the LED is formed, nitrogen (N) is doped on the light emitting layer as an isoelectronic trap in addition to dopants to attain normal carrier concentration.